The present invention generally relates to the fabrication of integrated circuits. More particularly, the invention provides an improved throttle valve and improved methods for controlling pressure within a processing chamber.
High density integrated circuits, commonly termed VLSI devices, are typically formed on semiconductor wafers by subjecting the wafers to a number of deposition, masking, doping and/or etching processes. The wafers are placed onto a pedestal or susceptor within a process chamber and process gas(es) are delivered into the chamber onto the wafer to perform the various deposition and etching steps. For example, one typical process involves delivering SiH.sub.4 and N.sub.2 into the process chamber while applying RF energy to form a plasma for depositing silicon nitride on the wafer. During each step, once the appropriate layers have been deposited and/or removed from the wafer, the remaining plasma and gas residue are withdrawn from the process chamber by a suitable vacuum source, such as a pump.
An important consideration in semiconductor processing is the gas pressure within the process chamber. For example, the gas pressure within the process chamber typically affects the characteristics of the layers deposited on the wafer and/or the geometry of the portions etched from the wafer. Small changes in the chamber pressure during processing may lead to non-uniform deposition or etching on the wafer, which is typically undesirable.
The gas pressure within a semiconductor process chamber is usually controlled by a throttle valve disposed along a discharge line between the gas outlet of the process chamber and the pump. The throttle valve is coupled to a pressure sensor, such as a manometer, and an external controller that causes the throttle valve to open and close to regulate the pressure within the chamber. Existing throttle valves usually include a valve body with a through-hole in communication with the discharge line and a plug rotatably disposed within the valve body. The plug is rotated within the valve body by a suitable drive motor to vary the cross-sectional area of the through-hole, thereby regulating the gas flow therethrough and the pressure within the process chamber.
During certain processes, such as high pressure process steps, the throttle valve is substantially closed so that a low gas flow-rate is established through the valve. In this substantially closed position, a surface of the plug blocks a portion of the through-hole and, therefore, is typically exposed to the process gases that are discharged from the chamber along the discharge line. These gas particles bombard the exposed surface of the plug and may adhere and solidify on the plug surface, thereby forming a thin deposition layer on the plug surface.
Gas deposition on the exposed surface of the plug may cause problems with the operation of the throttle valve. For example, the valve plug typically contacts sealing surfaces on the valve body in a substantially frictionless manner as it rotates within the valve body. The sealing surfaces prevent process gases from leaking between the valve body and the plug. However, the gas deposition build-up on the plug surface increases the friction between the plug and the valve body sealing surfaces. This increased friction applies a larger load on the drive motor and may stretch the drive belt that couples the plug shaft with the motor, eventually causing the belt to break. This reduction in the lifetime of the belt increases the downtime of the apparatus, and therefore decreases the throughput of the manufacturing process.
In addition, the valve plug motor is typically configured to rotate in small steps, with each of these steps corresponding to a distinct chamber pressure. This step configuration is considered desirable because it allows the operator to control the chamber pressure by rotating the valve plug a specific number of steps corresponding to the pressure desired. Increased friction between the plug and the valve body may cause the motor to skip steps, thereby disturbing the one-to-one correspondence between the motor steps and chamber pressure intervals. When this occurs, the motor steps will not correspond to their associated pressure settings, which could cause the operator to set an inaccurate chamber pressure during processing.
Another problem with existing throttle valves is that process gases passing through the valve body may leak between the sealing surfaces of the body and the plug. The gases swirl around within the valve, abrading the sealing surfaces of the valve body and forming grooves within these surfaces. These grooves may eventually become large enough to allow process gas to leak through the throttle valve. Gas leakage through the throttle valve will generally cause a reduction in the process chamber pressure, which can have adverse effects on the desired characteristics of the semiconductor wafer.
What is needed in the semiconductor manufacturing industry, therefore, are improved methods and apparatus for controlling the gas pressure within a process chamber. These methods and apparatus should be designed to inhibit gas deposition and subsequent solidification on the throttle valve, thereby improving operation of the motor and increasing the lifetime of the valve, which increases the throughput of the process. In addition, these methods and apparatus should be capable of minimizing leakage of gas through the valve body to effectively maintain the desired pressure within the process chamber.